Steering valves of this generic class are commonly used in vehicle power steering systems. The construction of such steering valves is known. A steering input shaft is connected by a centering spring, such as a torsion bar, to an output shaft connected with a pinion which meshes with a rack. Movement of the rack effects steering of the vehicle. It is known to use a rotary valve which, as a result of rotation of the input shaft relative to the output shaft, either connects or disconnects hydraulic fluid channels which terminate in bores leading to either a hydraulic fluid pump or a hydraulic cylinder. When the input shaft is rotated relative to the output shaft, hydraulic fluid under pressure is supplied to the hydraulic cylinder which assists movement of the rack in one of two possible directions. Activation of such a steering valve occurs by rotation of the input shaft relative to the output shaft wherein a torsional moment resulting from the torsion bar must be overcome.
In one known steering valve, a valve bushing rotates with the output shaft and surrounds the input shaft. The valve bushing has control channels which, as a control means, can be made to coincide with bores which pass through a housing in order to direct hydraulic fluid.
Another known steering valve is a rotary disk steering valve which has channels which lie essentially axially and which are made to coincide with each other. Such a rotary disk steering valve is known from DE 42 09 647 A1, wherein a valve bushing connected to the input shaft is surrounded by a valve bushing connected to the output shaft. The valve bushings overlap in a region where annular surfaces on each valve bushing lie next to each other. Each valve bushing has bores located in the region of their respective annular surfaces. The bores are connected to hydraulic fluid lines. By rotating one of the valve bushings relative to the other valve bushing, the bores are caused to either coincide or separate, which leads to a connection or separation of the corresponding fluid lines. Similar rotary disk steering valves are known from DE 28 33 009 A1 and DE 42 32 570 C1.
The known rotary disk steering valves require a multitude of additional components in order to form the rotary valve. This causes the size of these steering valves to be undesirably large and their manufacture to be very costly. Due to the use of valve bushings, guiding the hydraulic fluid to the annular surfaces is problematic. It is frequently necessary to form bores which intersect multiple times with each other. Such bores can also be connected to each other by diagonal channels. Since it is necessary to provide a minimum tolerance for a valve having a control bushing, and since the bushing is deformed during operation under the pressure of the hydraulic fluid which can lead to an increase in friction, the known rotary valves are expensive to manufacture. During assembly, the adjustment of elements, such as valve bushings, which move relative to each other is also very costly because a corresponding hydraulic seal must be established with each valve bushing.
Finally, it is known to construct rotary valves with annular disks on the annular surfaces. Control edges on the annular disks are formed by bores or elliptical formations which lie on arcs of a circle. A relative rotation of the annular surfaces results in a variable overlapping of opposing circular bores, which is minimal at first but which increases with the amount of rotation. The initially limited overlapping of the control edges is known to lead to noise due to hydraulic fluid flow, which is frequently found to be a disadvantage.
In order to improve this type of rotary disk steering valve such that the same can be manufactured with few components and limited expense in a compact design, it is known from DE 195 05 384 A1 to form a valve in the region of the opposing end surfaces of the input shaft and output shaft. In this configuration, it is necessary to form opposing end surfaces on the input and output shafts in order to form the rotary valve. Axially extending control channels are formed in the opposing end surfaces of the shafts. Additional components are omitted and the size of the valve is reduced.
Regardless of whether the valve is formed by a valve bushing or by control channels provided in the region of opposing end surfaces of the input and output shafts, the known valves are subject to the disadvantage that the characteristic control features are not variable. Once a valve has been manufactured, its control characteristics cannot be varied, particularly in response to functional vehicle parameters, such as vehicle speed. As disclosed in DE 4,330,338 C1, an additional problem of the known rotary disk steering valves is that automatic control of a steering valve can be realized only at great expense for components and for a control means for controlling the steering valve.